I'm having the same exact problem. More often at cold starts. I know theyre not exactly connected to each other but i only started hearing the rattles just after i came back from the dealers. here's what happened to me.
i have 32k miles. 2011 lci e90 335i. Just recently, i started hearing it after i came back from the dealers with a faulty dmtl pump (got my car back 11/15). 2nd time this has happened in 3 weeks. the check engine light wouldnt turn off even after the first replacement. i ended up leaving my car at the shop for a whole week. The light doesn't come on anymore but i hear the rattle. (side note: my car is completely stock)
so there's no way it could've been from mods.
Here's the tech report if it helps you in any way. I'm a bit concerned and it doesn't seem like they know what the problem is.
anyone else having dmtl pump problems? and possibly to rattles?